A local area network ("LAN") is a high-speed data communication system that electronically links computers and other data processing devices located within a small geographic area. Such small geographic areas may include a workgroup, a department of a company, or a single floor of a multistory building. Among other useful functions, LANs enable users to share vital computing resources, such as servers, printers, and CD-ROM drives. LANs also can serve as a secure broadcast domain for users connected to the LAN.
Data processing devices in a LAN exchange both data and control information in conformance with fixed protocols that define the operation of the network. One such commonly used class of network protocols is the "Carrier Sense Multiple Access with Collision Detection" protocol (CSMA/CD). The CSMA/CD protocol is defined in ANSI/IEEE Std. 802.3, published by the Institute of Electrical and Electronics Engineers, Inc., 345 East 45th Street, New York, N.Y., 10017, and commonly referred to as "the IEEE standards."
Ethernet is one widely-used CSMA/CD protocol, having several types such as 10Base-T and 100Base-TX which also are defined in IEEE standard 802.3. 100Base-TX technology is an improved version of 10Base-T technology, which has been in widespread use for many years. Specifically, the 10Base-T protocol of IEEE standard 802.3 is concerned with transferring data at a rate of 10 megabits per second over twisted-pair copper cables. Consider, for example, a LAN having two computers that can transfer data at 10 megabits per second only. Before one of the computers (the transmitting computer) can begin transmitting data to the other (the receiving computer), the transmitting computer first must establish that there is a 10Base-T communication link associated with the receiving computer. This typically is accomplished by means of link pulses that each computer transmits directly after power-up during out-of band periods (i.e. the time when the computers are not transmitting data packets and control information related to the data packets). The link pulses, commonly termed "normal" link pulses, consist of 100 nanosecond pulses every 16 milliseconds +/- 8 milliseconds. After power-up, the transmitting computer monitors the link for normal link pulses. When the transmitting computer receives a predetermined number of normal link pulses, thereby indicating the presence of a link that is capable of transmitting data to the receiving computer at the 10Base-T rate, the transmitting computer begins transmitting data across the LAN to the receiving computer.
The 100Base-TX protocol (commonly referred to as "Fast Ethernet") expands the IEEE standard 802.3 for 10Base-T communications to allow data to move at an effective rate of 100 megabits per second through twisted pair copper cables of the type that typically are used for 10Base-T transmissions. The 100Base-TX protocol therefore transmits data at 10 times the rate of existing 10Base-T systems and thus, is more desirable to use than the existing 10Base-T protocol. Many existing computer systems and data transfer devices, however, only have a network interface for 10Base-T transmissions. Such devices consequently cannot utilize the improved 100Base-TX protocol. The art has responded to this problem by developing upgraded network interfaces (commonly referred to as "10/100 interfaces") to existing 10Base-T network interfaces that enable data transfer devices to interface with networks using either 10Base-T Ethernet or 100Base-TX Ethernet.
10/100 interfaces can be connected to the Peripheral Component Interconnect (PCI) bus in a data transmission device such as, for example, a personal computer. A 10/100 interface has a data receiving input port that connects to both an encoder/decoder device and a filtering device. Both the encoder/decoder device and filtering device are connected to a PCI bus interface. When a 10Base-T signal is received by the input port, for example, the filtering device filters noise out of the signal and transmits it to the PCI bus interface. Conversely, when a 100Base-TX signal is received at the input port, the encoder/decoder processes the signal and then transmits it to the PCI bus interface. The 10Base-T signal is not transmitted by the encoder/decoder and the 100Base-TX signal is not transmitted by the filtering device.
It is known that the filtering device, which typically is connected in parallel with the encoder/decoder, must be configured to eliminate any distorting effect on a 100Base-TX input signal as it is transmitted between the input port and the encoder/decoder. A first known solution requires a buffering device, manufactured from active electronic elements, to be connected between the filtering device and the encoder/decoder. This enables the filter to receive the 10Base-T signal without distorting a 100Base-TX signal transmitted into the encoder/decoder. The additional active buffering device, however, increases the cost of the overall device. Another known solution requires that filtering device be manufactured from active electronic elements. One such active filter used for these purposes is the Pericom p15L100 chip, available from Pericom Semiconductor Corporation of San Jose, Calif. Similar to devices using an active buffering device, however, a filter manufactured from active electronic elements typically is expensive to manufacture and, consequently, increases the overall cost of the 10/100 interface.
Accordingly, it would be desirable to provide an inexpensive filtering device for a 10/100 interface device that is constructed to effectively filter noise from a 10Base-T signal when a 10Base-T signal is received, but does not distort a 100Base-TX signal transmitted between the network input port and the encoder/decoder when a 100Base-TX signal is received.